A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a vehicle is equipped with a navigation function to guide a user to a destination through a calculated route. Such a navigation system detects the position of the user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium, for example, a CD-ROM (compact disc read-only memory), a DVD (digital versatile disc), or a hard disc, or from a remote data server.
Typically, the navigation system displays a map image on a monitor screen while superimposing thereon, in a highlighted manner, a calculated route to the destination and a mark representing the current vehicle position. When the vehicle moves toward the specified destination, the current vehicle position changes on the calculated route. At a cross section, if a turn is necessary, the navigation system notifies the user which direction to turn at the intersection by an arrow image on the monitor screen as well as by a voice instruction.
The navigation is designed to establish a preferable route to a specified destination such as a shortest route, a quickest route, etc. for a route guidance operation. The user is able to specify a destination through various input methods of the navigation system. As is known in the art, one of the input methods for specifying a destination is an “Intersection” method.
In this method, a user is able to specify an intersection as a destination by inputting two cross streets in the navigation system. For instance, an intersection formed by “Irvine Center Drive” and “Jeffery Street” can be a destination where the user inputs the names of these two streets. In calculating a route to such an intersection, however, in some cases, a navigation system of today may fail to create an optimum calculated route.
FIGS. 1A-1B are schematic diagrams showing an example of such a situation where an intersection 11 is selected as a destination through the “Intersection” method noted above to reach there from a current position 18. In the example of FIG. 1A, the intersection 11 is formed by streets 15, 17, 19, and 20 where each of the streets 19 and 20 is a one-way street as indicated by arrows 21A and 21B, respectively. The streets 15 and 17 are normal streets where vehicles can travel in two ways.
The schematic view of FIG. 1B shows a route 13A that is a calculated route created by the navigation system for the route guidance between the current position 18 and the intersection destination 11. As seen from FIG. 1B, the calculated route 13A includes road segments A-D, and it is clear that it is not an ideal route. This is because the calculated route 13A takes the road segment A, then makes a left turn to take the road segment B, then to the road segments C and D to reach the intersection destination 11 which is obviously an undesirable detour.
In the situation of FIG. 1A, a better route would be a simple straight line from the current position 18 to the intersection destination 11 via the street 17 as shown in FIG. 1C. The calculated route 13B in FIG. 1C is a straight line between the current position 18 and the intersection destination 11 with the shortest distance. In contrast, the calculated route 13A in FIG. 1B makes a left turn, and two right turns to reach the intersection destination 11 which is inefficient and redundant.
FIGS. 2A-2C show another example of situation where the navigation system may create a calculated route to an intersection destination which is an inefficient and redundant route. In the example of FIGS. 2A-2C, an intersection destination 31 is a cross point of a street 35 and a street 37. It is assumed that the navigation system calculates a route between a current vehicle position 38 and the intersection destination 31.
In the example of FIG. 2B, the navigation system has created a calculated route 23A which includes road segments A-D between the current vehicle position 38 and the intersection destination 31. The calculated route 23A will pass the intersection destination 31 via the road segment A on the route 23A. The calculated route 23A then makes a right turn to the road segment B, and makes still another right turn to the road segment C, and finally makes a right turn to the road segment D to reach the intersection destination 31. Clearly, the route 23A is inefficient and a route 23B in FIG. 2C is a desirable route.
Such an inappropriate route may be created because of the structure of the map data currently available in the market in which an intersection is defined by a plurality of road segments (link-spot pairs) each being a one way road. As known in the art, a street in the map data used in the navigation system is defined by a plurality of road segments of links. The road segments are connected with one another at a spot (node) where the spot (geometric point data) specifies the absolute location of the connection point of the two road segments.
When an intersection is a destination, because of the structure of the map data and the route calculation process, the navigation system may fail to produce the most preferable route as described in detail later. Thus, it is desired to overcome the shortcomings in the conventional technology described above to obtain more efficient routes without changing the contents of the map data.